#include<iostream>
#include<cmath>
#include<cstdio>
using namespace std;
double a,b,c;
void chuli()
{
	scanf("%lf %lf %lf",&a,&b,&c);
	double p = (a+b+c)/2;
	double mianji = sqrt(p*(p-a)*(p-b)*(p-c));
	printf("%.2lf\n",mianji);	
}
int main()
{
	chuli();
	return 0;
}
